Although the present invention may be used in relation to various multiphase fluids, i.e. oil containing bubbles or other particles, in order to detect the presence of liquid in a supply line or other conduit, the invention will be described in relation to the detection of water induction in a steam operated power plant.
The induction of water or cool steam vapor onto turbine elements in a steam operated power plant can cause severe damage requiring immediate and costly repair, such as plastic deformation, broken blades, cylinder distortion and broken bolts. In most instances such damage is immediately recognizable and extremely costly to repair. Even if such damage is not of a degree rendering it immediately recognizable and thus requiring immediate repair, it nonetheless can be very costly in two respects. First, such damage will extend power plant down time for routine scheduled inspection and subsequent repair, and second such damage can cause a decrease in power plant performance. It has been estimated that over a twelve month period a decrease in performance of one third of one percent of a 500 MW power plant can cost $200,000.00. Consequently, there is a need for equipment which can detect water induction before an impending incident.
Detection of Water Induction in Steam Turbines, Phase 3: Field Demonstration, Electric Power Research Institute (EPRI), Project 637-2, Final Report EPRI CS-4285 dated September, 1985, describes various monitoring systems for detecting water induction in a steam operated power plant. One such system involved the use of temperature sensing devices, such as thermocouples, positioned in so-called thermocouple wells located along the length of supply lines from which induction might occur, for example an extraction line which leads to a low-pressure feedwater heater. A predetermined drop in line temperature would constitute an indication that water or cool steam vapor was present in the supply line. The problem with such monitors was in the response time associated with the thermocouples. For certain situations, thermocouples are too slow to provide adequate warning of an impending water-induction incident.
Another monitoring system described in the EPRI Report was said to utilize electronic liquid-level sensors to detect the presence of water in the subject supply lines. The problem with these monitors is the time delay exhibited due to piping flow restrictions.
A third type of monitoring system was described in the EPRI Report and was also described in Barton, Serge P., et al., Results of a Field Verification for a Turbine Water Induction Monitor, presented at the EPRI Seminar on Fossil Plant Retrofits for Improved Heat Rate and Availability, December 1-3, 1987 in San Diego, Calif. This third system described positioning an ultrasonic acoustic transmitter on one side of a turbine extraction line which leads to a low-pressure feedwater heater. An ultrasonic acoustic receiver was said to be positioned on the opposite side of the extraction line and in so called line-of-sight propagation alignment with the transmitter. The presence of water would enhance the transmission of a line-of-sight propagated signal to the receiver. In the absence of water, no line-of-sight signal would be received, although, a significantly smaller strength ultrasonic signal would still travel through the wall of the supply line to the receiver. Since the strength of the signal traveling through the wall is easily distinguishable from the line-of-sight signal, the presence or absence of water could be determined. The only delays associated with this type of system are those resulting from the generation, transmission, reception and processing of the ultrasonic signal. Such delays were said to be in an acceptable range in order to give adequate warning of an impending incident. Unfortunately, certain operational conditions were masked from detection by the ultrasonic monitoring system.
In normal operation, extraction lines and steam lines are conduits for vapor flow. At such times condensate in heaters and drain receivers are at or near their saturation temperatures. During rapid plant transients, such as when the load is rapidly reduced, a sudden drop in pressure in the feedwater heater can occur, lowering the saturation temperature of the feedwater, which in turn causes the immediate formation of steam bubbles at the vapor pressure of the liquid. The liquid enthalpy will be reduced by the heat of vaporization, e.g., bubble formation, until thermal equilibrium is restored. Since bubbles are being formed throughout the liquid, the bulk volume of the liquid increases. The increasing bulk volume results in the movement of liquid and bubbles to lower pressure regions of the power plant, i.e. back through one or more extraction lines into the turbine casing and rotating elements.
The presence of bubbles in a conduit incorporating the previously described ultrasonic monitor cause a scattering of the line-of-sight ultrasonic signal. As a result, the monitor does not sense the impending water induction incident. Consequently, a need exists for a monitoring system which exhibits the speed of the ultrasonic monitoring system and is capable of detecting a water induction incident in a supply or extraction line, despite the presence of bubbles.